The present invention relates to automatic dynamic compression and expansion systems, particularly for use in sound recording and reproducing systems.
It is known that the dynamic range of much program material is considerably greater than can be accommodated by currently available sound recording and reproducing systems. For example, orchestral music in a concert hall may have a dynamic range as great as 85 db, whereas an average phonograph system, of which the record is the limiting component, can only accommodate a dynamic range of perhaps no more than 65 db. Compander systems are known in which the program signals are compressed in amplitude before they are recorded, and are expanded in complementary fashion in the reproducing process so as to restore the original dynamic range to the signal. It is also known that the process of compression and expansion increases the perceived signal-to-noise ratio by reducing the effect of noise originating from the record disc (or other recording medium) on the signal being processed.
Another approach to compansion is the so-called Dolby B audio noise reduction system, developed primarily to improve the reproduction of tape cassettes. Such systems comprise a summing amplifier receiving, at one input, the audio signal to be transmitted over the noisy channel, and at the other summing input, a signal from a network which compresses a band of high frequencies by a factor varying inversely with the audio signal in that frequency band. This system, by providing two summed signals to the noisy channel, one a straight through signal, has a non-linear characteristic on the decibel scale. The philosophy of this approach is that the straight through signal is dominant at high signal levels and minimizes the distortion resulting from high level signals and transitions between low level and high level signals. Expansion of the signal recovered from the noisy channel is keyed to the level of the recovered signal. By compressing the band of high frequencies according to the signal content of this frequency band, and by using a logarithmically non-linear compression technique, significant frequency distortion can result if the signal level recovered from the noisy channel is not carefully adjusted to be identically the same as the signal level applied to the channel.
While the Dolby B system has enjoyed acceptance in the recorded tape field, the technique is generally unacceptable for use in the recording and reproduction of phonograph records, particularly when it may be desired to reproduce the record without first expanding the recorded compressed signal. That is, unless the signals compressed according to the Dolby technique (or by other systems which in the compression process separate the components of the source signal into two or more frequency bands) are expanded before reproduction, the reproduced signal will be badly altered in frequency response and artistically unacceptable.
Another known companding system that provides a great deal of noise reduction is one manufactured by dbx Company, the details of which are described in U.S Pat. Nos. 3,681,618, 3,789,143 and 3,714,462. This system, like Dolby B, is optimized for high frequency noise reduction; the compressor and expander function over the audio frequency spectrum, but the control signal is strongly pre-emphasized so that high frequencies control the gain. Among the reasons that this companding system is undesirable for use with phonograph records, particularly when it may be desired to reproduce the record without expansion, is that the high frequencies controlling the gain have short time constants and therefore cause fast gain change, or "pumping", in the compressor and the expander. It also means that with a high frequency signal on the record controlling the gain of the expander, there is low frequency preemphasis on playback which causes drastic modulation of "rumble" on the record. The dbx system was designed primarily for tape use and is not optimized for disc record use. Also, it is not "compatible" (in the sense that a disc record containing a signal compressed with a dbx compressor can be acceptably reproduced without first expanding the signal), nor does it have proper noise reduction characteristics for use on disc records.
Another known noise reduction system is the "High-Com" compander manufactured by Telefunken, Hannover, Germany, the details of which are described in a Telefunken publication dated April 1979. This system is similar in operation to the dbx system except that the compression and expansion characteristics have break points at high and low signal amplitude levels beyond which there is no compression or expansion. This alleviates some of the "pumping" that is encountered when it is attempted to use the "High-Com" system in the recording and reproduction of phonograph records, but otherwise it has most of the above-outlined drawbacks of the dbx system.
In summary, although all three of these currently best known noise reducing companding systems, namely, Dolby B, dbx and the Telefunken system, provide significant signal-to-noise improvement in the applications for which they were designed, neither is acceptable for use in the recording and reproduction of disc records, particularly when it may be desired to reproduce, without first expanding, a compressed record. In particular, the compression process of each of these systems sufficiently alters the original information signal that unless it is complementarily expanded before reproduction the results are artistically unacceptable.
Of the companders heretofore proposed for use in the recording and reproduction of sound signals on disk records, the most relevant to the present invention of which applicant is aware is that described in commonly assigned U.S. Pat. Nos. 3,197,712 to Bauer, and 3,230,470 to Kaiser. One embodiment of the Bauer system automatically modifies the gain of the useful signal transmission channel in response to signal volume level. Modification of the characteristic does not take place continuously as the signal volume varies but, rather, is effected in such fashion as to permit a given range of signal level variation during which the characteristic remains constant before modification of the characteristic occurs. One way of accomplishing this is to provide for continuous modification of the gain with increasing signal levels, but no modification with decreasing signal levels until the signal level has dropped a predetermined amount, thus providing a "platform" at which the characteristic is not changed. Kaiser U.S. Pat. No. 3,230,470 describes a system embodying the principles of the Bauer patent, but utilizing alternative circuitry for automatically modifying the gain characteristic of the signal transmission channel. More particularly, the transmission channel incorporates a variable gain amplifier responsive to a control signal for modifying the gain of the amplifier and means for developing such control signal by charging a capacitance through diodes connected in parallel to provide oppositely directed paths for current to and from the capacitor, respectively. The diode in the charging path is biased so as to remain non-conducting until the voltage input to the diodes has risen a predetermined amount above the bias voltage; the fixed bias employed for this purpose establishes the limits of the "platform" or range of variation in signal for which no modification of the transmission channel characteristic takes place.
Although these systems, by virtue of effecting gain modification in a linear mode, with provision for a signal level range "platform" in suitable extent at each signal level in which the gain does not change, to a significant extent accomplish gain modification without perceptible deterioration of the intelligence carried by the signal, they do not respond properly to certain types of signals encountered in the recording of music, for example, and cause a certain amount of deterioration of the information signal discernible to listeners who, in general, have become more discriminating in the interim since these systems were introduced. Moreover, these systems do not mask noise introduced by the record medium as effectively as desired by discriminating listeners. Further, if the channel contains impulse noise, which a record does, the impulse can change the expander gain indefinitely, with wholly unacceptable results.
It is an object of the present invention to provide a compression-expansion system which makes it possible to compress the dynamic range of a signal, with attendent masking of record medium noise, yet enabling the intelligence carried by the signal to be acceptably reproduced without first expanding the compressed signal, and when the reproduced signal is subjected to complementary expansion, substantially the entire dynamic range can be restored with highly effective masking of record surface noise. Stated another way, the primary object of the invention is to provide a compander system the compressor of which is capable of compressing the dynamic range of a program signal to enable its being recorded on a phonograph record or other noisy medium and to achieve effective masking of the medium noise, yet is compatible with uncompressed records in that the sound signal reproduced therefrom, even without expansion, differs very little from the source signal, and the expander of which, if used, is capable of restoring substantially the entire dynamic range with highly effective masking of record surface noise.